1. Field of the Invention
The present invention relates to a full-color toner for oil-less fixing suitable for use in an image forming device using electrophotographic technology such as a full-color copier, full-color printer, and so forth that employs oil-less fixing.
2. Description of the Related Art
Dry developers suitable for use in the above image forming devices are roughly classified into two-component developers in which toner is mixed with a carrier such as ferrite powder, iron powder, glass beads, and so forth, magnetic single-component developers in which magnetic powder is comprised in the toner itself, and non-magnetic single-component developers. The toners used for these developers have a binder resin and a colorant as the main components, while also containing a wax for ensuring satisfactory low-temperature fixability to the recording sheet, release agent for preventing offset, charge control agent for imparting polarity (positive charge or negative charge), and so forth. After these materials are mixed at prescribed ratios, the toner is manufactured as a powder after undergoing steps such as melt-kneading, pulverizing and classifying, and finally subjected to surface treatment in which an external additive such as silica, titanium oxide, alumina or various types of resin fine particles is adhered to control fluidity, chargeability, cleaning properties and storage properties, etc. and ultimately provided as the developer.
In the fixing device of these image forming devices, an oil such as silicone oil having satisfactory releasing properties has been coated onto the fixing roller to prevent so-called offset, that is, toner adheres and accumulates on the fixing roller and other fixing members. However, since this method requires an oil tank and oil coating device, the device becomes complex and large. In addition, since this method also causes deterioration of the fixing roller, maintenance is required at fixed intervals. Moreover, since the adherence of oil to copy paper and OHP (overhead projector) film and so forth cannot be avoided, there is the problem of poor color tone due to adherence of oil in the case of OHP film in particular.
In consideration of the above problems, a so-called oil-less type of image forming device has come to be provided in recent years that does not use release oil in the fixing device for the purpose of simplifying maintenance, conserving resources, reducing costs and so forth. Instead of using release oil, measures have generally been employed in which a release agent like wax is added in large amounts within the toner particle or the molten elastic modulus of the binder resin is enhanced by crosslinking or containing high molecular weight components, to supplement the function of the release oil.
In addition, relative to full-color images, there is a considerable demand for photographic, glossinessy images, and in order to respond to this demand, the toner face after fixing is required to be smooth, and the toner must have high transparency. Consequently, it is necessary that the toner have extremely low viscosity at the fixing temperature. However, in order to lower the viscosity of the toner at the fixing temperature extremely low, it is necessary to decrease the molecular weight of the binder resin. However, lowering the molecular weight brings about a decrease in the durability of the resin in the developing device, resulting in the problem of the rapid occurrence of streaked image unevenness, an increased degree of background fogging and so forth.
However, in the case of image forming devices that employ an oil-less fixing system using a type of toner that contains a large amount of release agent as described above, problems such as defective image characteristics due to the occurrence of black spots (BS) due to filming on the photosensitive member or the occurrence of fusing to developing or charging members (developing roller, layer thickness regulating member, etc.) tended to occur easily during the course of printing a large number of sheets. In addition, although expanding the molecular weight distribution of the binder resin or increasing the molten viscosity by crosslinking is effective for solving the above problems, this causes unevenness in melting of the binder resin at the fixing temperature, and carries fatal problems for full-color toners, such as decreased smoothness of the image face, decreased image glossiness, inadequate optical transmittance of OHP images and other.
As described above, the occurrence of BS on the photosensitive member and fusing to the developing or charging members is caused by the addition of a large amount of waxes as release agents in the toner particles. On the other hand, inadequate glossiness of image surfaces and inadequate optical transmittance of OHP images are caused by restricting the amount of waxes added in the toner particles, and expanding of the molecular weight distribution of the binder resin to alleviate the above problems.
As there are also aspects of full-color toners that require high-quality images equivalent to ordinary silver halide photographs, and based on the need for image glossiness, color mixing property (color reproduction property) and transparency, polyester resins having sharp melting characteristics have been used, and waxes have been finely dispersed within a range that prevents the occurrence of BS on the photosensitive member and the occurrence of fusing to developing members and so forth. However, the range of added amount of waxes that solves the both problems is narrow, and the selection is not easily. Therefore, in order to finely disperse a large amount of wax, natural wax and polar wax are commonly used. As polyester resins inherently had poor environmental characteristics, presented difficulties in obtaining a stable charging amount relative to environmental changes such as temperature and humidity, tended to carry exacerbation of background fogging at high temperatures and high humidity and decreased image density at low temperatures and low humidity. Moreover, the use of natural wax or polar wax tended to cause these environmental characteristics to further worse.